Printers are commonly used to produce hard copy output in a variety of situations. One type of printer is the thermal printer. A thermal printer may operate as a direct thermal printer, in which images are formed on a specially treated paper or other media by applying heat directly to the surface of the media. Another type of thermal printer is the thermal transfer printer, in which an image is formed by applying heat to a ribbon, which causes the transfer of wax and/or resin from the ribbon to the media. In addition, thermal printers that are capable of operating as direct thermal or thermal transfer devices are available.
Corresponding to the two main types of thermal printer technologies, there are two main categories of thermal printer media; media adapted for use in direct thermal printers and media adapted for use in thermal transfer printers. Media adapted for use in a direct thermal printer is specially treated. In particular, media for direct thermal printers typically has a coating that changes color as heat is applied. Accordingly, no ribbon is required. However, the media has a relatively short shelf life. In addition, the image produced by the print process is limited to the capabilities of the media. Also, the media is sensitive to degradation from exposure to heat, the outdoors, or other harsh environments. Media that is adapted for use in connection with thermal transfer printers must receive ink that has been released from a print ribbon using heat. Accordingly, such media is generally adapted to have good ink receptivity. However, the media itself is not heat sensitive. As a result, the media has a relatively long shelf life. In addition, the color of the output is not as limited as for direct thermal printers. Where a thermal transfer process is used, it is desirable to control tension in the print ribbon in order to ensure acceptable print quality. Also, it is desirable to monitor the amount of unused print ribbon that remains available. However, the ability to maintain ribbon tension and to monitor remaining print ribbon has been limited.
Media comprises a substrate on which the image is formed. Media for thermal printers, whether direct thermal or thermal transfer, may comprise a substrate made from a variety of materials, such as paper, films, or foils. In addition, the substrate of the media may be either unsupported or pressure sensitive. Unsupported substrate refers to any substrate that does not have a backing. A pressure sensitive substrate typically comprises a label adhered to a backing.
Examples of applications in which thermal printers have become prevalent include the ski industry, which commonly uses thermal printers to produce tickets at the point of sale on a durable label media. Another example is automotive service labeling, in which reminder labels for oil changes or other periodic maintenance procedures may be printed out on demand and placed in a customer's windshield. Examples of general business applications that use thermal printers include archive data labeling, asset inventory tracking, retail pricing, and media record tracking. Another example is the health care industry which uses thermal printer technology in connection with laboratory sample identification, patient identification, pharmacy labeling, x-ray tracking, etc. In addition to including textual information or graphics, labels often include machine-readable barcodes.
Depending on the use of the output being produced by the thermal printer, output of different sizes may be desirable. Most thermal printers use rolls of media. Accordingly, media comprising an unsupported substrate can be cut to an appropriate length after printing, either manually or automatically. Therefore, a thermal printer loaded with media having an unsupported substrate can produce output on pieces of media having different lengths without requiring that the media be changed. However, producing an output on media of a different width requires that the media loaded into the printer be changed, or that a different printer with media of the desired width already loaded be used. For media comprising labels, perforations and/or pre-printed matter, it is important to ensure that the media is properly registered with respect to the print head. Also, such registration should be easily established and reliably maintained for a variety of media widths. The task of establishing and maintaining proper registration is complicated where a user desires to load media of different widths at different times in a printer. For example, mechanisms for indexing print media have typically required that a user register an optical source and an optical sensor with one another and with indexing marks on the media. As a result, the indexing of print media has often been unreliable and difficult.
In addition to being available in different types and widths, media is available in different roll sizes. For example, rolls of relatively large diameter are desirable for stationary applications where large print volumes are anticipated. Smaller rolls can be used where print volumes for that media are relatively small or where it is desirable to use a compact printer. However, printers have been limited in their ability to accommodate media rolls of different sizes. In particular, compact printers have been unable to accommodate relatively large roll sizes. As a result, the ability to use a wide range of media roll sizes has been limited to relatively large, mid-range or industrial printers.
Thermal printers typically provide heat to the media and/or print ribbon using a plurality of elements spread across the media supply path as part of a print head. By way of illustration, a line across the width or most of the width of a piece of media can be formed by energizing the elements simultaneously for an instant of time. A line along the length of the media can be formed by energizing a single element (or a number of adjacent elements to produce a thicker line) for a period of time as the media is moved past the print head. Because individual elements of a print head can fail at different times, operators often accept diminished output quality rather than incurring the expense of replacing the entire print head. Therefore, it would be desirable to provided undiminished (or less diminished) print quality even when one or a few print elements have failed. It also would be desirable to detect print elements that are in the process of failing, so that remedial action can be taken.